The present invention relates to a method for prediction of electrical characteristics of an electrochemical storage battery.
It may be desirable to determine or to predict at any given time the state of an electrical storage battery, such as the state of charge or the heavy-current load capability. By way of example, the capability of a starter battery to start a motor vehicle with an internal combustion engine is governed by the state of charge and the state of aging and by the characteristic drop in capacity of the battery, since the current level which can be drawn from the starter battery and the power which can be emitted are limited. It is particularly important to determine the state of charge and the starting capability of a starter battery in situations in which, for example, the engine is operated intermittently hence, in this case, the vehicle power supply system together with its loads is still operated during times in which the engine is switched off, even though the generator is not producing any electrical power. In situations such as this, the monitoring of the state of charge and of the starting capability of a storage battery must ensure that the energy content of the storage battery always remains sufficient to still start the engine.
Widely differing methods are known for measurement of the state of charge and for determination of the load behavior of storage batteries. For example, integrated test equipment (amp-hour (Ah) meters) is used for this purpose, with the charging current being taken into account, and possibly with an assessment using a fixed charging factor. Since the usable capacity of a storage battery is highly dependent on the magnitude of the discharge current and the temperature, even a method such as this does not allow a satisfactory statement to be produced about the usable capacity which can still be drawn from the battery.
By way of example, in the case of a method for measurement of the state of charge, DE 22 42 510 C1 discloses the assessment of charging current by means of a factor which is itself dependent on the temperature and on the state of charge of the battery.
DE 40 07 883 A1 describes a method in which the starting capability of a storage battery is determined by measurement of the battery terminal voltage and of the battery temperature, and by comparison with a state of charge family of characteristics which is applicable to the battery type to be tested.
DE 195 43 874 A1 discloses a calculation method for the discharge characteristic and remaining capacity measurement of a storage battery, in which the current, voltage, and temperature are likewise measured, and with the discharge characteristic being approximated by means of a mathematical function with a curved surface.
DE 39 01 680 C1 describes a method for monitoring the cold starting capability of a starter battery, in which the starter battery is loaded with a resistance at times. The voltage which is dropped across the resistance is measured, and is compared with empirical values in order to determine whether the cold starting capability of the starter battery is still sufficient. The starting process is in this case used to load the starter battery.
Furthermore, DE 43 39 568 A1 discloses a method for determination of the state of charge of a motor vehicle starter battery, in which the battery current and the rest voltage are measured, and are used to deduce the state of charge. The battery temperature is also taken into account in this case. The charging currents which are measured during different time periods are compared with one another, and are used to determine a remaining capacity.
DE 198 47 648 A1 describes a method for learning a relationship between the rest voltage and the state of charge of a storage battery in order to estimate the storage capability. A measure for the acid capacity of the electrolyte in the storage battery is determined from the relationship of the rest voltage difference to the amount of current transferred during the load phase. This makes use of the fact that the rest voltage for the higher state of charge ranges which are relevant for practical use rises approximately linearly with the state of charge.
One problem of determining the state of an electrochemical storage battery using known methods is that wear occurs in particular while rechargeable storage batteries are being discharged and charged, as well as while they are being stored without any load, and the conventional methods do not take account of all the relevant wear factors.
In the case of lead-acid rechargeable batteries, the wear relates on the one hand to corrosion phenomena, which reduce the voltage level when heavy electrical loads are applied, and on the other hand to changes in the morphology and the chemical composition of the active substances. Furthermore, parasitic reactions such as electrolysis and corrosion of gratings, or else simple vaporization, lead to a loss of water from the electrolyte. In the case of a rechargeable battery with liquid electrolytes, this is evident in a reduction on the electrolyte level. Parts which were previously covered with electrolytes in consequence become exposed, and this can lead to a change in the corrosion behavior in these areas. Furthermore, acid stratification can occur by the acid falling in layers on the base of the storage battery, which leads to an increase in the acid capacity in the lower area and to a reduction in the acid capacity in the upper area. In the case of rechargeable batteries with solid electrolytes (e.g., so-called sealed rechargeable batteries), in which the electrolyte is immobilized, for example, by means of a glass fiber mat or a gel, the saturation level of the electrode set (which comprises the porous electrodes and microporous separators and/or a gel) falls with the electrolyte. This is evident, inter alia, in an increased internal resistance and, in some cases, in a reduced capacity. Furthermore, as the electrode set dries out to an increasing extent, the rate of the parasitic oxygen circulation rises, which, in the case of rechargeable batteries of this type, on the one hand reduces the water loss by electrolysis, but on the other hand can reduce the charging efficiency and can increase the heating of the rechargeable battery during charging.
In both situations, the rest voltage of the rechargeable battery for a given degree of discharge (DoD) rises, because the loss of water (WL) with the amount of sulfuric acid unchanged leads to an increased concentration of the dilute sulfuric acid electrolyte, and the rest voltage (U00) rises strictly monotonically with the acid concentration, by virtue of the electrochemical relationships.
Accordingly, it would be advantageous to provide an improved method for prediction of electrical characteristics of an electrochemical storage battery.